1. Field of the Invention
The present invention relates to a fuel metering system for turbine engines.
More specifically, the present invention relates to a highly accurate fuel metering system with a low leakage piston pump wherein fuel metering is accomplished by using a variable speed electric motor to drive the piston pump. Fuel flow is directly proportional to motor speed. With a brushless DC motor driving the piston pump speed can be accurately controlled.
2. Description of the Background Art
Many systems are known for controlling the supply of fuel in gas turbine engines and other types of engines. The typical gas turbine engine control system is complex and expensive if high fuel metering accuracy is required. In U.S. Pat. No. 3,447,315 to Long, a very early type of fuel system for gas turbine engines is described. A pump delivers fuel to an engine under the control of a throttle valve. A shut off valve is opened and the throttle valve permits passage of the fuel to a starter burner. There is no piston pump being driven by a DC motor to meter the fuel.
U.S. Pat. No. 3,946,551 to Linebrink et al relates to an electromechanical pumping and fuel metering device interfacing an electronic fuel control which is adapted for small engines. Linebrink et al comprises an engine driven centrifugal pump, a suitable metering pump, which is an electrically driven vane type of pump, and a pressure regulating valve. Fuel from a reservoir is delivered to a burner section of the power plant. The system is designed to permit the vane pump to pressurize the fuel to the required value during engine start up until idle speed is reached. This device is too small to be used with turbine engines and there is no piston pump driven by a speed controlled electric motor to meter fuel.
In U.S. Pat. No. 4,048,964 to Kissel, a closed loop, electronically controlled and regulated fuel metering system is described that operates an internal combustion engine. The system controls the mass ratio of fuel and air supplied. The air flow and fuel flow signals are measured by the fluid flowmeters and combined to cause the fuel to be delivered by a pump. The pump is a variable speed motor pump wherein the actual amount of fuel delivered and sensed by the flowmeter corresponds to the amount of fuel required to maintain the desired fuel-air ratio set by the controller. The effect of the motor current will cause the fuel pump to increase the fuel flow rate and the amount of the fuel supplied to the engine. However, the pump is not a piston pump and it is not driven by a speed controlled electric motor wherein the piston pump meters fuel to the turbine engine.
U.S. Pat. No. 4,208,871 to Riple, Jr. is directed to a fuel control system having a boost pump for pumping liquid fuel to a positive displacement pump which in turn meters the fuel to a fuel nozzle of a combustion engine. The system includes a fuel control system having independently operated pumping and metering devices. The metering device is controlled by a variable speed electric motor. An electronic control module controls the motor. When the engine requires low fuel volumes at relatively low pressures, the variable speed motor drives the pump at a relatively slow speed, and a substantial amount of fuel is supplied by the boost pump is bypassed to the fuel supply tank. At higher fuel demand and pressure conditions, the motor speed is increased, and the boost pump is not bypassed. In other words, the use of a piston pump to meter fuel to a turbine engine is not disclosed.
In U.S. Pat. No. 4,815,278 to White, an electrically driven fuel control system for a gas turbine engine is described. The system utilizes a combination of variation in power applied to an electrically powered drive of a positive displacement fuel pump and braking to achieve an optimized response characteristic approaching that of mechanically driven fuel systems directly driven from the gas turbine engine. In the preferred embodiment, for a decrease in fuel delivery exceeding a predetermined limit control of the electric drive for the fuel pump is achieved by braking. For decreases in fuel delivery having a magnitude less than the predetermined limit and for fuel increases, control is achieved by varying the power applied to the electric drive for the fuel pump. This is a different system than a piston pump driven by a brushless motor where the piston pump is used to meter the fuel to the turbine engine.
U.S. Pat. No. 4,830,576 to Patrick includes a metering fuel pump to feeding fuel containing vapor or air to an engine that includes a housing assembly. More specifically, the metering fuel pump attempts to discharging the fuel vapor out of the pump. A motor controller section controls the speed of the motor by regulating the flow of fuel. The motor controller section includes a brushless variable speed electric motor and a two-stage pump. The two-stage pump has a primary stage and secondary stage. The stages include a stationary cavity with a gerotor pump that includes an internal and external gear. The two-stage pump section imparts pressure to the fuel and encloses a cyclonic vapor separator section. Still, this system does not have a piston pump with a very high volumetric efficiency and very low leakage so that an amount of fuel pumped per revolution of the piston pump can be a known constant.
Published Application No. US2001/0054290 to Herbison et al. describes a fuel system for providing a desired fuel flow to an engine comprises a metering assembly for providing a variable portion of metered fuel flow to an associated nozzle system. The variable portion corresponds to a variable demand signal received by the metering assembly. A centrifugal pumping assembly is adapted to receive fuel from an associated pumping system. The centrifugal pumping assembly is capable of pressurizing and delivering adequate fuel flow to the metering assembly. A throttling valve is in fluid communication between the metering assembly and the centrifugal pumping assembly to maintain a constant pressure rise across the metering assembly. An electronic engine control provides the variable demand signal to the metering assembly. This system does not include a piston pump with a very high volumetric efficiency and low leakage so that the amount of fuel pumped per revolution of the piston pump is a known constant.
Published Application No. US 2002/0038540 to Griffiths et al. has a control system for use in controlling fuel supply to at least two sets of burners of an engine. The control system comprises two or more fuel pumps, each of the pumps being driven, in use, by a respective variable-speed motor. A control arrangement is provided for controlling the speed of the motors so as to regulate the rate of flow of fuel to each of the sets of burners. The use of two motors is inefficient in both cost and power.